Manipulation of the object to place it in a desired position in
within a very narrow range of tolerance.
In a precision process such as the manufacturing of LSIs, size and
position of fine patterns need to be detected accurately. One popular method to do this is the
optical probe method, in which a finely focused laser beam is used as the optical probe to scan the
surface of the specimen. By detecting the light scattered at the edges of the fine patterns and
processing the signals, the position of the pattern edges can be determined at a precision of 0.01
μm or better. Moving the specimen on the X-Y table enables the width and length of a
two-dimensional pattern to be measured. Moire and interference are used to align the wafer and the
reticle mask. To exploit the moire effect for alignment, a fine grating is drawn on both the wafer
and the mask. When the two gratings are placed close to each other, moire fringes appear, which are
detected and used for alignment. For higher precision, the gratings are irradiated with laser light
and the resulting interference fringes are used to check positional error. This method attains an
accuracy of about 0.01 μm. Precise driving mechanisms are also needed, and piezoelectric actuators
or hydrostatic screws are utilized in special cases.